Some techniques for detecting objects in captured images based on visual images are known in the art. For example, there is a method for calculating optical flows from captured sequential images and detecting a part of image corresponding to an object within area having same motion components. Since this can easily detect a moving object in the image, many object detection apparatus employs such method (for example, Japanese unexamined patent publication (Kokai) No. 07-249127)
However, when an imaging device for capturing images is moving (for example, when the imaging device is mounted onto an automobile or the like), it would be difficult to detect the moving object in the image accurately because some optical flows associated to the self-motion of the device is generated in the image. In such cases, if a motion field of the entire view associated to the self-motion are removed from the optical flows, the moving object in the image may be detected more accurately. For example, in Japanese unexamined patent publication No. 2000-242797, a motion detection method is disclosed where a variable diffusion coefficient is used when detecting optical flows in the image by means of a gradient method. According to this method, the diffusion coefficient is not fixed as in the conventional arts but compensated under some conditions, thereby noise resistance may be improved and differential of optical flows around object boundaries may be emphasized.
According to the method mentioned above, optical flows of the moving object, which is detected relatively easily, may be calculated accurately. However, when a stationary object on a stationary background is observed from a self-moving unit, it is difficult to segregate optical flows of the stationary object from that of the background. In this case, since the stationary object on the stationary background is recognized as a part of the background, optical flows are not emphasized and therefore the stationary object cannot be detected accurately.
Therefore, there is a need for an object detection apparatus and method capable of detecting stationary objects accurately based on images captured by a self-moving unit.